<!DOCTYPE html>
<html lang="en">
<head>
    <meta http-equiv="content-type" content="text/html;charset=UTF-8">
    <title>Title</title>
</head>
<body>

<!--遮罩层-->
<div style="position:fixed;background-color: black;top: 0;bottom: 0;right: 0;left: 0;opacity: 0.5;"></div>

<div style="height: 2000px;background-color: aquamarine">11111111111</div>

</body>
</html>